rgw/iam: simplify match_policy() for colon-delimited use only

rgw/iam: simplify match_policy() for colon-delimited use only

remove unused MATCH_POLICY_STRING

ARN::match() was the only caller for MATCH_POLICY_ARN, but it was used
to match the 'region' and 'account' components that were already parsed
out of the colon-separated ARN string. for that use, we don't need the
loop-over-colons behavior of match_policy() so can call match_wildcards()
directly

after doing the same for MATCH_POLICY_RESOURCE, we no longer have any
non-looping callers of match_policy() so can treat 'bool colonblocks' as
unconditionally true

Signed-off-by: Casey Bodley <cbodley@redhat.com>